Given:(3t + 9s) (3t – 9s),it is a product of 2 binomial expressions which have the terms :(3t + 9s) and (3t – 9s).
Now when we compare these expressions with the identities, we
find it in the form of (a-b)(a + b),where a = 3t and b = 9s,
The identity (a-b)(a + b) = a2-b2,
Hence, (3t + 9s) (3t – 9s) = (3t)2-(9s)2 = 9t2-81s2
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